@0x/contracts-test-utils: Export a custom describe() instead of

extending Mocha's ambient declarations.
`@0x/contracts-exchange`: Use `describe.optional` instead of
`blockchainTests.optional` in `test/fill_order.ts`.

contracts/test-utils/src/index.ts

@@ -45,5 +45,5 @@ export {
     Token,
     TransactionDataParams,
 } from './types';
-export { blockchainTests, BlockchainTestsEnvironment } from './mocha_blockchain';
+export { blockchainTests, BlockchainTestsEnvironment, describe } from './mocha_blockchain';
 export { chaiSetup, expect } from './chai_setup';